Kasper Dupont wrote:
> Is there a race condition in this piece of code from do_fork in
> linux/kernel/fork.c? I cannot see what prevents two processes
> from calling this at the same time and both successfully fork
> even though the user had only one process left.
>
> if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur
> && !capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE))
> goto bad_fork_free;
>
> atomic_inc(&p->user->__count);
> atomic_inc(&p->user->processes);
I don't see any locking in the call chain leading to this function, so
I think you're right. The attached patch fixes this. It costs an
extra 2 atomic ops in the failure case, but otherwise just makes the
processes++ operation earlier.
Patch is against 2.5.27, but applies against 30.
-- Dave Hansen haveblue@us.ibm.com
--- linux-2.5.27-clean/kernel/fork.c Sat Jul 20 12:11:07 2002 +++ linux/kernel/fork.c Fri Aug 2 09:35:17 2002 @@ -628,13 +628,15 @@ goto fork_out; retval = -EAGAIN; - if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur) { - if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE)) + atomic_inc(&p->user->processes); + if (atomic_read(&p->user->processes) > p->rlim[RLIMIT_NPROC].rlim_cur) { + if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE)) { + atomic_dec(&p->user->processes); goto bad_fork_free; + } } atomic_inc(&p->user->__count); - atomic_inc(&p->user->processes); /* * Counter increases are protected by
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